Most wire rope users like plastic impregnated wire ropes, such as disclosed, for example in U.S. Pat. No. 4,667,462 belonging to the present applicant and called Cushion Rope®, because such ropes are clean and their plastic exterior minimizes sheave and drum wear. The additional benefit is that the plastic impregnation also increases rope fatigue life.
On the other hand, conventional plastic impregnation of ropes is not a very controlled process since it is difficult to control how the rope is impregnated. Usually, the plastic just fills randomly the voids of the interior of the rope. During this process, it is difficult to avoid situations whereby two or more outer rope strands contact each other or the outer rope strands contact the outer strands of the core when the rope is flexed during use. These contact points become steel-to-steel abrasion points during the operation of the ropes, leading to the eventual failure of the rope.
Maintaining the strand-to-strand and strand-to-core separation in a plastic impregnated rope is quite difficult. One such method is disclosed in applicant's Canadian Patent Application No. 2,393,220 where specially designed plastic bands are provided to prevent contact between the wires of the core and those of the outer strands. However, this system requires a precise operational control and still leaves the possibility of contact between the wires of the outer strands.
In applicant's Canadian Patent No. 2,041,206 there is described a wire rope in which the core is provided with a plastic jacket and with wormings or spacers laid on the circumference of the jacketed core between the outer strands, so as to form separations between outer strands. The wormings are then normally compressed by the outer strands so as to fill the voids that exist between each pair of adjacent outer strands and the core. These types of ropes provide improved fatigue life, however, they are not fully plastic impregnated since they have only the core that is jacketed with plastic. For this reason, customers who wish to benefit from the cleanliness and low sheave and drum abrasion provided by fully plastic impregnated ropes, are reluctant to use such core jacketed ropes, also called Cushion Core® ropes, even when they are provided with wormings or spacers to improve their fatigue life.
Initial attempts at filling the outer strand voids of Cushion Core® ropes, whether they were provided with wormings or not, were unsuccessful mostly because the strand-to-strand plastic filling was frequently lost early in the life of the rope due to the flexing of the rope in use.
There is consequently a need to find a way whereby Cushion Core® ropes could be improved so as to provide their outer strands with plastic filling which would not fall-off during operation of the rope and which would increase the rope fatigue life over a similar fully plastic impregnated ropes.